In an NAS (network attached storage) system, it is an especially important function to set user permissions relating to documents stored in the system. There are a number of software tools available for setting user permissions. For example, P.R. China patent application No. 01122441.x entitled “System and Method for Setting User Permissions in a File System” discloses a system and method for setting user permissions. The system uses a permission verifying module and a permission setting module to set the user permissions of a document or part of the document in accordance with data on user IDs (identification codes) and user permission grades. The system can enhance efficiency in managing files, and improve security in a file system. However, the system can not quickly access the user permissions of documents in an access control list.
For example, the procedure of accessing a document comprises the following steps: verifying whether a user has a request process permission in relation to the document; and permitting the user to process the document if the user has the request permission. The procedure of verifying comprises the steps of: querying a user permission of a root directory in which the document is stored; querying the user permissions of lower directories one by one; and querying the user permission of the document. In this procedure, a cache memory can only store the latest semi-result of querying, and can not use other semi-results of querying. One user permission may be repeatedly queried. In addition, the procedure of querying takes too much time.